416 The American Naturalist, [May, 
favorable to its physiological processes. Whether the animal has 
any “preferences” or exercises any conscious choice is an open 
question ; but this question aside, the assumption that it is stimu- 
lated to move towards the light by the invisible heat-rays is 
clearly without foundation. The light, before impinging upon 
. the animal, must as a rule traverse a considerable thickness of 
water, by which the heat-rays are almost wholly absorbed, and 
thus rendered inoperative. Experimentally the same result is 
given as follows: If in the winter season an aquarium be placed 
close to a north window, in a warm room, the animals collect as 
usual on the light side, although, as shown by a thermopyle, the 
other sides may receive a much greater supply of heat-rays. Ex- 
periments with Bunsen flames or heated objects placed close to the 
aquarium and kept in a fixed position for days show no percep- 
tible movement of the Hydras towards the source of heat, pro- 
vided no luminous rays are given off from it. The most con- 
vincing evidence is afforded by the behavior of Hydras towards 
rays that have passed through water as compared with rays that 
have passed through liquids absorbing the same amount of heat 
but transmitting fewer light-rays. Thus it is easy to arrange 
an apparatus such that a group of Hydras is offered the choice 
between rays that have passed through water (transparent to the 
visible rays, but nearly impervious to heat-rays) and a strong 
solution of iddine which, as shown by the thermopyle, is practi- 
cally the same as water in respect to the transmission of 
heat-rays, but absorbs a large proportion of the visible rays. 
Under these circumstances the Hydras invariably move ™ the 
direction of the rays that have traversed the water, thus p 
ing that the attractive influence must be exerted by the visible 
rays. ; 
It is certain, therefore, that notwithstanding their complete lack 
of definite visual apparatus, both species of Hydra are not only 
very sensitive to the visible rays, but pe 
response to the stimuli afforded by them. 
that the heliotropism cannot have the same part to p 
life of green plants, since it is not peculiar to 
In this regard Hydra differs strikingly from 
rform definite actions if a i 
It seems certain, 2% 
the green Hydra 
the Protozoa, s 
